Patent-Document 1 and Patent-Document 2 disclose respectively examples of information-acquisition devices, each of which can acquire both of picture-information and optical-communication signal information. The information-acquisition devices disclosed in those documents use a CMOS image sensor, and their features lie in a structure in which a unit cell implementing one pixel has both of a function of acquiring the picture-information and a function of acquiring the optical-communication signal information. FIG. 18 shows an outline of a circuit configuration of the cell disclosed in those documents.
As shown in FIG. 18, a cell 100 of the earlier information-acquisition device encompasses a photodiode 111, a reset-transistor 114, a buffer circuit 115, a read-out switch SW100, two of mode-exchanging switches SW200, 300, a current amplification circuit 120, a picture-information output-signal line SL100 and an optical-communication signal information output-signal line SL110. By the way, the junction capacitance of the photodiode 111 serves as a charge-accumulation capacitor 112. In the earlier information-acquisition device, the picture-information acquired by the cell 100 is provided through the picture-information output-signal line SL100 to an output OUT2. The optical-communication signal information acquired by the cell 100 is provided through the optical-communication signal information output-signal line SL110 to an output OUT1.
In the picture-information acquiring-mode of the earlier information-acquisition device, when the mode-exchanging switch SW200 is turned off, the mode-exchanging switch SW300 is connected to a ground (GND) level. When a reset signal VRST in a pulse waveform is fed to a gate electrode of the reset-transistor 114, the reset-transistor 114 is turned on, and a charge quantity of the charge-accumulation capacitor 112 is initialized in response to a reset voltage VR. After the initialization, the reset-transistor 114 is turned off. Thus, the interconnection on the input side of the buffer circuit 115 (the interconnection is typically formed by a floating diffusion layer in many cases) is turned into floating state. For this reason, an input voltage VFD of the buffer circuit 115 is varied in accordance with the charge quantity accumulated in the charge-accumulation capacitor 112. In the picture-information acquiring-mode, as the on and off sates of the read-out switch SW100 are exchanged, a charge accumulation period and a charge transfer period are exchanged. The period in which the read-out switch SW100 is in the off state is the charge accumulation period, and the period in which the read-out switch SW100 is the on state is the charge transfer period. In the charge accumulation period, when light is irradiated to the cell 100, charges are generated in the photodiode 111. When the generated charges are accumulated in the charge-accumulation capacitor 112, the input voltage VFD of the buffer circuit 115 is decreased. When the read-out switch SW100 is turned on after the elapse of a predetermined period, the input voltage VFD (or an amplification voltage based on the input voltage VFD) is transferred through the buffer circuit 115 to the picture-information output-signal line SL100. Consequently, a voltage signal based on the picture-information is provided to the output OUT2.
In the optical-communication-signal information-acquiring mode of the earlier information-acquisition device, as the mode-exchanging switch SW200 is turned on, the mode-exchanging switch SW300 is connected to the side of the current amplification circuit 120. Moreover, in the optical-communication-signal information-acquiring mode, both of the reset-transistor 114 and the read-out switch SW100 are controlled to be in off sates. In the off state of the reset-transistor 114 and the read-out switch SW100, an optical communication signal is irradiated to the cell 100, and the charges are generated in the photodiode 111, and a very weak diode current flows. The current amplification circuit 120 amplifies the very weak diode current, and the amplified current is provided as an electric signal to the output OUT1. Consequently, a current signal based on the optical-communication signal information is provided to the output OUT1.